A pre-heat or furnace burner of the cyclone type can be described as an “adiabatic” circular burner having a combustion chamber into which combustion gas, such as air, is introduced tangentially to form a swirling flow. Fuel particles are introduced into the gas flow and by the centrifugal force acting on them they will be transported along the chamber wall. The fuel in a cyclone burner preferably comprises ground particles, but in comparison to a free standing solid fuel burner, the demand for fine material is much lower.
In many applications the temperature inside the cyclone burner is so high that the fuel ash melts and forms a slag, which must be continually withdrawn from the burner. This is typically the case when it is used to fire coal. In other applications, typically wood combustion, the temperature is controlled so that melted ash—stickiness—is avoided.
In most applications, the cyclone burner is refractory lined, preventing corrosion and minimizing heat losses. In combination with a high thermal density this leads to an approximately adiabatic temperature within the burner.
In many applications it is desirable to maintain the temperature within a certain temperature range in order to obtain a satisfactory carbon burnout while avoiding the drawbacks, such as the above mentioned stickiness, at high temperatures. The highest temperature is reached just below stoichiometric condition, i.e. the condition when the oxygen of the combustion gas or air added equals the amount for completely combusting the fuel. If less oxygen is added, i.e. sub-stoichiometric condition, the temperature will be lower, and the same applies if more oxygen is added, i.e. over-stoichiometric condition, since the excess oxygen will serve as a cooling medium. This is illustrated in appended FIG. 1.
The turndown ratio, i.e. the maximum to minimum operable fuel load ratio for a given cyclone burner, is limited by the demand of particle circulation and by extensive particle carryover (shortcutting). In other words, the gas flow or the velocity of the gas should be above a lower limit in order to entrain the fuel particles whilst avoiding disentraining them due to gravitational and frictional forces, and should also be below an upper limit in order to avoid particles exiting from the combustion chamber before being fully combusted.
The slagging cyclone burner is the most common application. They are operated in an over-stoichiometric condition, the main reason being to avoid a corrosive environment at reducing conditions when firing coals. Typically a turndown ratio of about 2:1 is possible. A slagging cyclone burner is used for complete melting of ash particles, which are mainly withdrawn as slag. In contrast, a non-slagging cyclone burner is operated at such conditions that severe slagging will not occur inside the burner. The ash is thereby mainly withdrawn as solid fly ash particles. Non-slagging cyclone burners can be operated under either sub- or over-stoichiometric conditions, although sub-stoichiometric is the most common. Typically, a turndown ratio of 4:1 is possible. Operation under sub-stoichiometric conditions is preferred because the burner can be built more compactly. The specific volume flow of gases through the cyclone burner (m3/kgfuel) can be regarded as approximately proportional to the stoichiometric ratio and thus a higher thermal load is possible under a sub-stoichiometric condition.
The prior art provides little controllability as regards the combustion process of cyclone burners, and it is difficult to achieve a larger turndown ratio than 4:1 while operating in the desired temperature range. The main reasons for this are because the retention time of the fuel particles inside the combustion chamber is limited at high gas flow or because the circulation in the combustion chamber becomes insufficient at low gas flow. One possible solution for obtaining a larger turndown ratio would be to provide a longer burner. However, such a construction would be costly, bulky and demand a lot of space. Furthermore, a longer burner would provide considerable layout difficulty if it was to replace a conventional existing burner.